Coordinated, reversible protein phosphorylation is a mechanism by which the activity of enzymes in cellular metabolic pathways is often regulated. Kinases and phosphatases catalyze the phosphorylation and dephosphorylation of substrates, respectively. A dynamic balance between the activities of kinases and phosphatases modulates a plethora of intracellular signaling pathways, including pathways responsive to hormones such as insulin and leptin. In the classic model, kinases act as positive regulators, whereas phosphatases serve to negatively regulate these signaling pathways; however, phosphatases have also been shown to positively regulate some pathways. Phosphatases are generally classified into three groups: the serine-threonine phosphatases, tyrosine phosphatases, and the dual-specificity phosphatases, the latter of which dephosphorylate serine, threonine and tyrosine phosphoamino acids (see, e.g., van Huijsduijnen et al., Drug Discov. Today, 2002, 7, 1013–1019).
Several proteins that bind to phosphorylated tyrosines, threonines, or serines have been identified; some are known as adaptor molecules for their presumptive role in mediating the physical interaction of signaling pathway components. Protein domains known as src-homology 2 (SH2) domains and phosphotyrosine binding (PTB) domains have been found to mediate the interaction between phosphorylated amino acid residues of activated plasma membrane-spanning receptor molecules and intracellular adaptor proteins. As one example, the classic adaptor molecule, growth factor receptor-bound protein 2 (GRB2), features a hallmark SH2 domain through which GRB2 interacts with phosphotyrosine-containing activated receptors. The potential antiproliferative effects of inhibiting signal transduction with the use of agents (small peptides and macrocycles) which specifically block the interaction between adaptor proteins and phosphorylated residues of activated receptors represents an active area of anticancer therapeutic research (see, e.g., Gao et al., Bioorg Med Chem Lett, 2001, 11, 1889–1892; Groves et al., Biochemistry, 1998, 37, 17773–17783; Liu et al., J Med Chem, 1999, 42, 3737–3741; Schoepfer et al., Bioorg Med Chem Lett, 2001, 11, 1201–1203; Yan et al., FEBS Lett, 2002, 513, 67–70).
Protein tyrosine phosphatases (PTPases) which reverse the phosphorylation of activated growth factor receptors are another means of attenuating signal transduction cascades. For example, autophosphorylation of the insulin receptor is essential for mediating the effects of insulin in a variety of cell types. Insulin signaling is initiated when insulin binds to extracellular subunits of the heterotetrameric insulin receptor tyrosine kinase (IRTK) and induces rapid autophosphorylation of several tyrosine residues in the intracellular part of the receptor, thus activating it. A fully phosphorylated IRTK transmits a signal to downstream cellular substrates, and activity of the phosphorylated IRTK can be reversed by dephosphorylation. The phosphotyrosine residues functioning as a control switch of IRTK activity appear to be tightly regulated by PTP-mediated dephosphorylation. The phosphatase PTP1B, first cloned from human placenta (Tonks et al., J Biol Chem, 1988, 263, 6722–6730), is believed to downregulate the insulin receptor (Faure et al., J Biol Chem, 1992, 267, 11215–11221; Zinker et al., Proc Natl Acad Sci U S A, 2002, 99, 11357–11362).
Insulin regulates important metabolic processes and plays a key role in control of blood glucose levels. The role of the adipocyte-derived hormone, leptin, appears to be the regulation of body mass, via suppression of food intake and increased energy expenditure. Type 2 diabetes mellitus and obesity are characterised by resistance to hormones insulin and leptin, attributed to attenuated or diminished receptor signaling. A large body of data from cellular, biochemical, mouse and human genetic and chemical inhibitor studies have identified PTP1B as a major negative regulator of both insulin and leptin signaling, implicating PTP1B in both insulin resistance and leptin resistance (Cheng et al., Dev Cell, 2002, 2, 497–503; Cook and Unger, Dev Cell, 2002, 2, 385–387; Ukkola and Santaniemi, J Intern Med, 2002, 251, 467–475; Zabolotny et al., Dev Cell, 2002, 2, 489–495).
Additionally, evidence suggests that insulin and leptin action can be enhanced by the inhibition of PTP1B. Because PTP1B dephosphorylates and thereby downregulates signaling by IRTK and the leptin receptor, specific inhibition, deletion or disruption of PTP1B function would be expected to lead to undiminished insulin receptor and leptin receptor signaling, and, therefore, insulin sensitivity and resistance to obesity. This hypothesis is strongly supported by the observation that mice lacking the PTP1B gene are generally healthy but exhibit increased insulin sensitivity and are able to maintain glucose homeostasis with about half the level of circulating insulin, making them resistant to diet-induced obesity. In response to insulin administration, PTP1B deficient mice exhibit a significant increase in insulin receptor phosphorylation in liver and muscle compared to wild type controls. The insulin-sensitive phenotype of the PTP1B knockout mouse is reproduced when the phosphatase is knocked down with an antisense oligonucleotide in obese mice (Zinker et al., Proc Natl Acad Sci U S A, 2002, 99, 11357–11362).
Inhibitors of PTPases, in general, are currently under intense study as potential therapeutic agents for treatment of obesity, diabetes, autoimmune diseases, infectious disease, inflammation, osteoporosis, cancer and neurodegenerative diseases. Because blocking PTPases with non-specific inhibitors results in massive and rapid stimulation of kinase-catalyzed phosphorylation cascades, the presiding assumption is that blocking individual PTPases could result in stimulation of specific pathways (van Huijsduijnen et al., Drug Discov Today, 2002, 7, 1013–1019). PTP1B appears to be a very attractive candidate for the design of pharmacological agents capable of inhibiting these negative regulator(s) of the insulin and leptin signaling pathways and therefore beneficial for the treatment of Type 2 diabetes and obesity. Furthermore, PTP1B has also been reported to regulate neurite extension mediated by cell-cell and cell-matrix adhesion molecules (Pathre et al., J Neurosci Res, 2001, 63, 143–150) and to be a major player in catalyzing the dephosphorylation and activation of c-Src in human breast cancer cell lines (Bjorge et al., J Biol Chem, 2000, 275, 41439–41446). Thus, PTP1B has emerged as an ideal target for these inhibitors, not only for the management or treatment of diabetes and obesity, but also cancer and neurodegeneration, (Asante-Appiah and Kennedy, Am J Physiol Endocrinol Metab, 2003, 284, E663–670; Ukkola and Santaniemi, J Intern Med, 2002, 251, 467–475; van Huijsduijnen et al., Drug Discov Today, 2002, 7, 1013–1019).
Some small molecules such as insulinomimetics, phosphotyrosine mimetics, substituted carboxylic acids, non-carboxylic acid-containing ligands, difluromethylphosphophonates, and hydroxamido vanadates have been studied as inhibitors of PTPases (Burke et al., Biochemistry, 1996, 35, 15989–15996; Faure et al., J Biol Chem, 1992, 267, 11215–11221; Jia et al., J Med Chem, 2001, 44, 4584–4594; Larsen et al., J Med Chem, 2002, 45, 598–622; Liu et al., J Med Chem, 2003, 46, 3437–3440). Many reported inhibitors of PTPases have been phosphorus-containing compounds, tight-binding inhibitors, and/or inhibitors that covalently modify the enzymes (Iversen et al., J Biol Chem, 2002, 277, 19982–19990; Leung et al., Bioorg Med Chem, 2002, 10, 2309–2323; Shen et al., J Biol Chem, 2001, 276, 47311–47319; Zhang et al., J Biol Chem, 2000, 275, 34205–34212). In a search for a general, reversible, competitive PTP inhibitor that could be used as a common scaffold for lead optimization for specific PTPs, 2-(oxalylamino)-benzoic acid (OBA) was identified and reported to be a competitive inhibitor of several PTPs (Andersen et al., J Biol Chem, 2000, 275, 7101–7108). However, as with many PTPase inhibitors, OBA exhibits a lack of specificity for inhibition of PTP1B. Thus, OBA has been used as a starting point in a screen for selective PTP1B inhibitors (Iversen et al., J Biol Chem, 2000, 275, 10300–10307).
Reports of various phosphatase inhibitors have been published in WO 04/062664; WO 04/041799; WO 03/82841; WO 03/092679; WO 02/18321; WO 02/18323; WO 02/18363; WO 03/37328; WO 02/102359; WO 02/04412; WO 02/11722; WO 02/26707; WO 02/26743; WO 01/16122; WO 01/16123; WO 00/17211; WO 00/69889; WO 01/46203; WO 01/46204; WO 01/46205; WO 01/46206; WO 01/70753; WO 01/70754; WO 01/17516; WO 01/19830; WO 01/19831; WO 98/27065; WO 00/53583; WO 99/11606; WO 03/32916; WO 01/16097; WO 98/27092; WO 98/56376; WO 03/33496; WO 99/58514; WO 99/58518; WO 99/58519; WO 99/58521; WO 99/58522; WO 99/61410; WO 97/40017; and U.S. Pat. Nos. 6,166,069; 6,310,081; 6,110,963; 6,057,316; 6,001,867; and 5,7983,74. Other compounds have been reported in U.S. 2003/0060419 U.S. 2004/0167188 and WO 98/53814.
Thus, PTP1B is an ideal therapeutic target for intervention in type 2 diabetes and obesity, as well as, neurodegenerative and anarchic cell proliferative diseases such as cancer, and there remains a long felt need for inhibitors of proteins that bind to tyrosine phosphonates, threonine phosphonates or serine phosphonates, and in particular, inhibitors of PTP1B with modified or improved profiles of activity.